Integrated LNT-TWC catalyst

ABSTRACT

A layered catalyst composite for the treatment of exhaust gas emissions, effective to provide lean NO x  trap functionality and three-way conversion functionality is described. Layered catalyst composites can comprise catalytic material on a substrate, the catalytic material comprising at least two layers. The first layer comprising rare earth oxide-high surface area refractory metal oxide particles, an alkaline earth metal supported on the rare earth oxide-high surface area refractory metal oxide particles, and at least one first platinum group metal component supported on the rare earth oxide-high surface area refractory metal oxide particles. The second layer comprising a second platinum group metal component supported on a first oxygen storage component (OSC) and/or a first refractory metal oxide support and, optionally, a third platinum group metal supported on a second refractory metal oxide support or a second oxygen storage component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)to U.S. Provisional Patent Application No. 61/968,669, filed on Mar. 21,2014, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention is directed to an exhaust gas purifying catalystand methods for its use. More particularly, the invention pertains to alayered exhaust gas purifying catalyst that is capable of executing botha NO_(x) absorbing function and a three-way conversion (TWC) function,the composite may be referred to as LNT-TWC. The exhaust gas purifyingcatalyst may be used to treat exhaust gas streams, especially thoseemanating from lean burn engines.

BACKGROUND

Emission of nitrogen oxides (NO_(x)) from lean burn engines must bereduced in order to meet emission regulation standards. Conventionalthree-way conversion (TWC) automotive catalysts are suitable for abatingNO_(x), carbon monoxide (CO), and hydrocarbon (HC) pollutants in theexhaust of engines operated at or near stoichiometric air/fuelconditions. The precise proportion of air to fuel which results instoichiometric conditions varies with the relative proportions of carbonand hydrogen in the fuel. An air-to-fuel (A/F) ratio of 14.65:1 (weightof air to weight of fuel) is the stoichiometric ratio corresponding tothe combustion of a hydrocarbon fuel, such as gasoline, with an averageformula CH_(1.88). The symbol λ is thus used to represent the result ofdividing a particular A/F ratio by the stoichiometric A/F ratio for agiven fuel, so that; λ=1 is a stoichiometric mixture, λ>1 is a fuel-leanmixture and λ<1 is a fuel-rich mixture.

Engines, especially gasoline-fueled engines to be used for passengerautomobiles and the like, are being designed to operate under leanconditions as a fuel economy measure. Such future engines are referredto as “lean burn engines.” That is, the ratio of air to fuel in thecombustion mixtures supplied to such engines is maintained considerablyabove the stoichiometric ratio so that the resulting exhaust gases are“lean,” i.e., the exhaust gases are relatively high in oxygen content.Although lean-burn engines provide advanced fuel economy, they have thedisadvantage that conventional TWC catalysts are not effective forreducing NO_(x) emissions from such engines because of excessive oxygenin the exhaust. Attempts to overcome this problem have included the useof a NO_(x) trap. The exhaust of such engines is treated with acatalyst/NO_(x) sorbent which stores NO_(x) during periods of lean(oxygen-rich) operation, and releases the stored NO_(x) during the rich(fuel-rich) periods of operation. During periods of rich (orstoichiometric) operation, the catalyst component of the catalyst/NO_(x)sorbent promotes the reduction of NO_(x) to nitrogen by reaction ofNO_(x) (including NO_(x) released from the NO_(x) sorbent) with HC, CO,and/or hydrogen present in the exhaust.

In a reducing environment, a lean NO_(x) trap (LNT) activates reactionsby promoting a steam reforming reaction of hydrocarbons and a water gasshift (WGS) reaction to provide H₂ as a reductant to abate NO_(x). Thewater gas shift reaction is a chemical reaction in which carbon monoxidereacts with water vapor to form carbon dioxide and hydrogen. Thepresence of ceria in an LNT catalyzes the WGS reaction, improving theLNT's resistance to SO₂ deactivation and stabilizing the PGM. NO_(x)storage (sorbent) components including alkaline earth metal oxides, suchas oxides of Mg, Ca, Sr, and Ba, alkali metal oxides such as oxides ofLi, Na, K, Rb, and Cs, and rare earth metal oxides such as oxides of Ce,La, Pr, and Nd in combination with precious metal catalysts such asplatinum dispersed on an alumina support have been used in thepurification of exhaust gas from an internal combustion engine. ForNO_(x) storage, baria is usually preferred because it forms nitrates atlean engine operation and releases the nitrates relatively easily underrich conditions. However, catalysts that use baria for NO_(x) storageexhibit a problem in practical application, particularly when thecatalysts are aged by exposure to high temperatures and lean operatingconditions. After such exposure, such catalysts show a marked decreasein catalytic activity for NO_(x) reduction, particularly at lowtemperature (200 to 350° C.) and high temperature (450° C. to 600° C.)operating conditions. NO_(x) storage materials comprising barium (BaCO₃)fixed to ceria (CeO₂) have been reported, and these NO_(x) materialshave exhibited improved thermal aging properties.

To meet current governmental regulations (for example, Euro 6),catalytic converters must effectively convert hydrocarbons at lowtemperatures during lean operation, and they must effectively converthydrocarbons and NO_(x) under conditions favoring stoichiometric exhaustgas. An additional challenge is storing nitrogen oxides during leanoperation and reducing these oxides during rich operation. Due to spacelimitations, however, using a separate TWC together with a separate LNTcatalyst is not ideal. Thus, there is a need for a technology thatbalances standard TWC activity with LNT functionality, while alleviatingthe space concerns that occur when a separate TWC catalyst is usedtogether with a separate LNT catalyst.

SUMMARY

A first embodiment pertains to a layered catalyst composite for anexhaust stream of an internal combustion engine, the layered catalystcomposite comprising a catalytic material on a substrate, the catalyticmaterial comprising at least two layers, wherein: the first layercomprises rare earth oxide-high surface area refractory metal oxideparticles, an alkaline earth metal supported on the rare earthoxide-high surface area refractory metal oxide particles, and at leastone first platinum group metal component supported on the rare earthoxide-high surface area refractory metal oxide particles; and the secondlayer comprises a second platinum group metal component supported on afirst oxygen storage component (OSC) and/or a first refractory metaloxide support and, optionally, a third platinum group metal supported ona second refractory metal oxide support or a second oxygen storagecomponent.

In a second embodiment, the layered catalyst composite of the firstembodiment is modified, wherein the catalyst is effective to provideboth lean NO_(x) trap functionality and three-way conversionfunctionality.

In a third embodiment, the layered catalyst composite of the first andsecond embodiments is modified, wherein the first layer is disposed onthe substrate that comprises a flow-through monolith and the secondlayer is disposed on the first layer.

In a fourth embodiment, the layered catalyst composite of the first andsecond embodiments is modified, wherein the second layer is disposed onthe substrate that comprises a flow-through monolith and the first layeris disposed on the second layer.

In a fifth embodiment, the layered catalyst composite of the firstthrough fourth embodiments is modified, wherein the substrate comprisesa wall-flow filter and the first layer is on an inlet set of passagesand the second layer is on an outlet set of passages.

In a sixth embodiment, the layered catalyst composite of the firstthrough fourth embodiments is modified, wherein the substrate comprisesa wall-flow filter and the first layer is on an outlet set of passagesand the second layer is on an inlet set of passages.

In a seventh embodiment, the layered catalyst composite of the firstthrough sixth embodiments is modified, wherein the layered catalystcomposite is free of hydrocarbon trap material.

In an eighth embodiment, the layered catalyst composite of the firstthrough seventh embodiments is modified, wherein the rare earthoxide-high surface area refractory metal oxide particles have a ceriaphase present in a weight percent of the particles in the range of about20% to about 80% on an oxide basis.

In a ninth embodiment, the layered catalyst composite of the firstthrough eighth embodiments is modified, wherein the rare earthoxide-high surface area refractory metal oxide particles compriseceria-alumina particles.

In a tenth embodiment, the layered catalyst composite of the firstthrough ninth embodiments is modified, wherein the ceria-aluminaparticles have a ceria phase present in a weight percent of theparticles in the range of about 20% to about 80% on an oxide basis.

In an eleventh embodiment, the layered catalyst composite of the firstthrough tenth embodiments is modified, wherein the ceria-aluminaparticles are substantially free of alkaline earth metal.

In a twelfth embodiment, the layered catalyst composite of the firstthrough eleventh embodiments is modified, wherein the first, second, andthird platinum group metal components independently comprise platinum,palladium, and/or rhodium.

In a thirteenth embodiment, the layered catalyst composite of the firstthrough twelfth embodiments is modified, wherein the first platinumgroup metal component comprises both palladium and platinum.

In a fourteenth embodiment, the layered catalyst composite of the firstthrough thirteenth embodiments is modified, wherein the first platinumgroup metal component comprises platinum.

In a fifteenth embodiment, the layered catalyst composite of the firstthrough fourteenth embodiments is modified, wherein the second platinumgroup metal component comprises palladium.

In a sixteenth embodiment, the layered catalyst composite of the firstthrough fifteenth embodiments is modified, wherein the third platinumgroup metal component comprises rhodium.

In a seventeenth embodiment, the layered catalyst composite of the firstthrough sixteenth embodiments is modified, wherein the first and secondrefractory metal oxide supports independently comprise a compound thatis activated, stabilized, or both selected from the group consisting ofalumina, zirconia, alumina-zirconia, lanthana-alumina,lanthana-zirconia-alumina, baria-alumina, baria-lanthana-alumina,baria-lanthana-neodymia-alumina, alumina-chromia, alumina-ceria, andcombinations thereof.

In an eighteenth embodiment, the layered catalyst composite of the firstthrough seventeenth embodiments is modified, wherein the first andsecond oxygen storage components comprise a ceria-zirconia composite ora rare earth-stabilized ceria-zirconia.

In a nineteenth embodiment, the layered catalyst composite of the firstthrough eighteenth embodiments is modified, wherein the first oxygenstorage component and the second oxygen storage component comprisedifferent ceria-zirconia composites, the first oxygen storage componentcomprising ceria in the range of 35 to 45% by weight and zirconia in therange of 43 to 53% by weight and the second oxygen storage componentcomprising ceria in the range of 15 to 25% by weight and zirconia in therange of 70 to 80% by weight.

In a twentieth embodiment, the layered catalyst composite of the firstthrough nineteenth embodiments is modified, wherein the alkaline earthmetal comprises barium.

In a twenty-first embodiment, the layered catalyst composite of thefirst through twentieth embodiments is modified, wherein the barium ispresent in an amount in the range of about 5% to 30% by weight on anoxide basis of the first layer.

In a twenty-second embodiment, the layered catalyst composite of thefirst through twenty-first embodiments is modified, wherein the secondlayer further comprises a second alkaline earth metal supported on thefirst refractory metal oxide support.

In a twenty-third embodiment, the layered catalyst composite of thetwenty-second embodiment is modified, wherein the second alkaline earthmetal comprises barium.

In a twenty-fourth embodiment, the layered catalyst composite of thetwenty-third embodiment is modified, wherein the barium is present in anamount in the range of about 0% to about 10% by weight on an oxide basisof the second layer.

In a twenty-fifth embodiment, the layered catalyst composite of thefirst through twenty-fourth embodiments is modified, wherein under leanconditions, the layered catalyst composite is effective tosimultaneously store NO_(x), and to oxidize CO, HC, and NO to NO₂.

In a twenty-sixth embodiment, the layered catalyst composite of thefirst through twenty-fifth embodiments is modified, wherein under richconditions, the layered catalyst composite is effective tosimultaneously convert CO and HC and to release and reduce NO_(x).

In a twenty-seventh embodiment, the layered catalyst composite of thefirst through twenty-sixth embodiments is modified, wherein understoichiometric conditions, the layer catalyst composite is effective tosimultaneously convert CO, HC, and NO_(x).

In a twenty-eighth embodiment, the layered catalyst composite of thefirst embodiment is modified, wherein the catalyst composite iseffective to provide both lean NO_(x) trap functionality and three-wayconversion functionality; the substrate comprises a flow-through carrierand the first layer is disposed on the substrate and the second layer isdisposed on the first layer; the rare earth oxide-high surface arearefractory metal oxide particles comprise ceria-alumina particles havinga ceria phase present in a weight percent of the composite in the rangeof about 20% to about 80% on an oxide basis; the first platinum groupmetal component comprises palladium and/or platinum; the second platinumgroup metal component comprises palladium; and the third platinum groupmetal component comprises rhodium.

A twenty-ninth embodiment pertains to an exhaust gas treatment systemcomprising the layered catalyst composite of the first throughtwenty-eighth embodiments located downstream of an engine.

In a thirtieth embodiment, the exhaust gas treatment system of thetwenty-ninth embodiment is modified, wherein the engine comprises a leanburn engine.

In a thirty-first embodiment, the exhaust gas treatment system of thethirtieth embodiment is modified, wherein the lean burn engine comprisesa lean gasoline direct injection engine.

In a thirty-second embodiment, the exhaust gas treatment system of thetwenty-ninth through thirty-first embodiments is modified, furthercomprising a catalyst selected from the group consisting of TWC, SCR,GPF, LNT, AMOx, SCR on a filter, and combinations thereof.

In a thirty-third embodiment, the exhaust gas treatment system of thetwenty-ninth through thirty-second embodiments is modified, furthercomprising an SCR catalyst located downstream of the layered catalystcomposite.

A thirty-fourth embodiment pertains to a method for treating a gascomprising hydrocarbons, carbon monoxide, and nitrogen oxidescomprising: contacting the gas with the layered catalyst composite ofthe first through twenty-seventh embodiments, wherein: under leanconditions, the layered catalyst composite is effective tosimultaneously store NO_(x), and to oxidize CO, HC, and NO; under richconditions, the layered catalyst composite is effective tosimultaneously convert CO and HC and to release and reduce NO_(x); andunder stoichiometric conditions, the layered catalyst composite iseffective to simultaneously convert CO, HC, and NO_(x).

A thirty-fifth embodiment pertains to a method of making a layeredcatalyst composite, the method comprising providing a carrier andcoating the carrier with first and second layers of catalytic material;the first layer comprising rare earth oxide-high surface area refractorymetal oxide particles, an alkaline earth metal supported on the rareearth oxide-high surface area refractory metal oxide particles, and atleast one first platinum group metal component supported on the rareearth oxide-high surface area refractory metal oxide particles, thesecond layer being the outermost layer of the composite, comprising asecond platinum group metal component supported on a first oxygenstorage component (OSC) or a first refractory metal oxide support and athird platinum group metal component supported on a second refractorymetal oxide support or a second oxygen storage component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a honeycomb-type refractory substratemember which may comprise a layered catalyst composite according to anembodiment;

FIG. 2 is a partial cross-sectional view enlarged relative to FIG. 1 andtaken along a plane parallel to the end faces of the substrate of FIG.1, which shows an enlarged view of one of the gas flow passages shown inFIG. 1;

FIG. 3A is a graph of the cycle NO_(x) conversion according to theExamples 1, 2, 3, 4, and 7 in fresh states;

FIG. 3B is a graph of the NO_(x) trapping capacity according to theExamples 1, 2, 3, 4, and 7 in fresh states;

FIG. 4A is a graph of the cycle NO_(x) conversion according to theExamples 1, 2, 3, 4, and 7 after aging at 950° C. for 5 hours in 2% O₂and 10% steam in N₂;

FIG. 4B is a graph of the NO_(x) trapping capacity according to theExamples 1, 2, 3, 4, and 7 after aging at 950° C. for 5 hours in 2% O₂and 10% steam in N₂;

FIG. 5A is a graph of the tailpipe NO_(x) emissions according to theExamples 1 and 4 after aging at 950° C. for 64 hours in an internalcombustion engine;

FIG. 5B is a graph of the NMHC emissions according to the Examples 1 and4 after aging at 950° C. for 64 hours in an internal combustion engine;

FIG. 6 is the TEM image of the undercoat of Example 1 in fresh state;and

FIG. 7 is the TEM image of the topcoat of Example 1 in fresh state.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

According to embodiments of the invention, provided is a layeredcatalyst composite for an exhaust stream of an internal combustionengine that balances TWC activity and LNT functionality. In leanoperation, the catalyst composite allows for conversion of carbonmonoxide (CO) and hydrocarbons (HC) and storage of NO_(x). In richoperation, the catalyst is effective to convert CO and HC and to releaseand reduce NO_(x). In stoichiometric operation, the catalyst compositeallows for simultaneous conversion of CO, HC, and NO_(x).

In one or more embodiments, a layered catalyst composite comprises acatalytic material on a substrate. The catalytic material comprises atleast two layers, a first layer and a second layer. The first layercomprises rare earth oxide-high surface area refractory metal oxideparticles, an alkaline earth metal supported on the rare earthoxide-high surface area refractory metal oxide particles, and at leastone first platinum group metal component supported on the rare earthoxide-high surface area refractory metal oxide particles. The secondlayer comprises a second platinum group metal component supported on afirst oxygen storage component (OSC) and/or a first refractory metaloxide support and, optionally, a third platinum group metal supported ona second refractory metal oxide support or a second oxygen storagecomponent

With respect to the terms used in this disclosure, the followingdefinitions are provided.

As used herein, the terms “catalyst” or “catalyst material” or“catalytic material” refer to a material that promotes a reaction. Asused herein, the term “catalyst composite” refers to a catalytic articleincluding a carrier substrate, for example a honeycomb substrate, havingone or more washcoat layers containing a catalytic material, forexample, a PGM component that is effective to catalyze oxidation of CO,HC, and NO.

As used herein, the terms “layer” and “layered” refer to a structurethat is supported on a surface, e.g. a substrate. In one or moreembodiments, the layered catalyst composite of the present inventioncomprises two distinct layers coated on a single substrate or substratemember, one layer (e.g., the first or the second layer) over top of theother (e.g., the second or the first layer). In one or more embodiments,the first layer is coated over the entire axial length of a substrate(e.g., a flow-through monolith) and the second layer is coated over theentire axial length of the first layer. In other embodiments, the secondlayer is disposed on a substrate, and the first layer is disposed on thesecond layer. In one or more embodiments, the first and second layersare washcoats.

As used herein, the term “washcoat” has its usual meaning in the art ofa thin, adherent coating of a catalytic or other material applied to acarrier substrate material, such as a honeycomb-type carrier member,which is sufficiently porous to permit the passage of the gas streambeing treated. As is understood in the art, a washcoat is obtained froma dispersion of particles in slurry, which is applied to a substrate,dried and calcined to provide the porous washcoat.

As used herein, the terms “refractory metal oxide support” and “support”refer to the underlying high surface area material upon which additionalchemical compounds or elements are carried. The support particles havepores larger than 20 Å and a wide pore distribution. As defined herein,such metal oxide supports exclude molecular sieves, specifically,zeolites. In particular embodiments, high surface area refractory metaloxide supports can be utilized, e.g., alumina support materials, alsoreferred to as “gamma alumina” or “activated alumina,” which typicallyexhibit a BET surface area in excess of 60 square meters per gram(“m²/g”), often up to about 200 m²/g or higher. Such activated aluminais usually a mixture of the gamma and delta phases of alumina, but mayalso contain substantial amounts of eta, kappa, and theta aluminaphases. Refractory metal oxides other than activated alumina can be usedas a support for at least some of the catalytic components in a givencatalyst. For example, bulk ceria, zirconia, alpha alumina, silica,titania, and other materials are known for such use.

One or more embodiments of the present invention include a refractorymetal oxide support comprising an activated compound selected from thegroup consisting of alumina, zirconia, alumina-zirconia,lanthana-alumina, lanthana-zirconia-alumina, baria-alumina,baria-lanthana-alumina, baria-lanthana-neodymia-alumina,alumina-chromia, ceria, alumina-ceria, and combinations thereof.Although many of these materials suffer from the disadvantage of havinga considerably lower BET surface area than activated alumina, thatdisadvantage tends to be offset by a greater durability or performanceenhancement of the resulting catalyst. As used herein, the term “BETsurface area” has its usual meaning of referring to the Brunauer,Emmett, Teller method for determining surface area by N₂ adsorption.Pore diameter and pore volume can also be determined using BET-type N₂adsorption or desorption experiments.

In one or more embodiments, the first and second refractory metal oxidesupports independently comprise a compound that is activated,stabilized, or both, selected from the group consisting of alumina,zirconia, alumina-zirconia, lanthana-alumina, lanthana-zirconia-alumina,baria-alumina, baria-lanthana-alumina, baria-lanthana-neodymia-alumina,alumina-chromia, ceria, alumina-ceria, and combinations thereof. Inspecific embodiments, the second refractory metal oxide comprisesalumina.

As used herein, the term “space velocity” refers to the quotient of theentering volumetric flow rate of the reactants divided by the reactorvolume (or the catalyst bed volume) which indicates how many reactorvolumes of feed can be treated in a unit time. Space velocity iscommonly regarded as the reciprocal of the reactor space time.

As used herein, the term “rare earth oxide-high surface area refractorymetal oxide particles” refers to a mixture of rare earth oxide and highsurface area refractory metal oxide particles that are employed as acarrier for catalytic components. In one or more embodiments, the rareearth oxide is selected from at least one oxide of a rare earth metalselected from Ce, Pr, Nd, Eu, Sm, Yb, and La, and mixtures thereof. Insome embodiments, the rare earth oxide can be mixed with one or moreother components such as lanthanum, praseodymium, neodymium, niobium,platinum, palladium, rhodium, iridium, osmium, ruthenium, tantalum,zirconium, hafnium, yttrium, nickel, manganese, iron, copper, silver,gold, gadolinium, and combinations thereof. In one or more embodiments,the high surface area refractory metal oxide comprises any high surfacearea refractory metal oxide known in the art. For example, the highsurface area refractory metal oxide can comprise one or more of alumina,zirconia, alumina-zirconia, lanthana-alumina, lanthana-zirconia-alumina,baria-alumina, baria-lanthana-alumina, baria-lanthana-neodymia-alumina,alumina-chromia, ceria, and alumina-ceria. In one or more embodiments,the rare earth oxide-high surface area refractory metal oxide particlescomprise ceria-alumina particles. In specific embodiments, theceria-alumina particles have a ceria phase present in a weight percentof the first layer in the range of about 20% to about 80% on an oxidebasis, including 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%,75%, or 80%. In one or more specific embodiments, the average CeO₂crystallite size of the fresh and aged samples, obtained from XRD, canbe used as a measurement for CeO₂ hydrothermal stability. Accordingly,in one or more embodiments, the CeO₂ is present in the form ofcrystallites that are hydrothermally stable and have an averagecrystallite size of less than 130 Å after aging at 950° C. for 5 hoursin 2% O₂ and 10% steam in N₂. In a specific embodiment, theceria-alumina particles include a ceria phase present in a weightpercent of the composite in an amount of about 50% on an oxide basis. Inother specific embodiments, the ceria-alumina particles include a ceriaphase present in a weight percent of the composite in an amount of about30% on an oxide basis.

In one or more embodiments, the CeO₂ is present in the form ofcrystallites that are hydrothermally stable and are resistant to growthinto larger crystallites upon aging at 950° C. As used herein, the term“resistant to growth” means that the crystallites upon aging grow to asize no larger than an average of 130 Å. In a specific embodiment, theCeO₂ crystallite size, as determined by XRD, after aging the catalyticarticle at 950° C. for 5 hours in 2% O₂ and 10% steam/N₂ is less than130 Å. According to one or more embodiments, the CeO₂ crystallite sizeof the powder samples and the coated catalysts are different. In thecoated catalysts, other washcoat components may have a stabilizationeffect on CeO₂. Therefore, after the same 950° C. aging, the CeO₂crystallite size of the coated catalyst is smaller than that of thepowder.

As used herein, the term “average crystallite size” refers to the meansize as determined by XRD described below.

As used herein, the term “XRD” refers to x-ray diffractioncrystallography, which is a method of determining the atomic andmolecular structure of a crystal. In XRD, the crystalline atoms cause abeam of x-rays to diffract into many specific directions. By measuringthe angles and intensities of these diffracted beams, athree-dimensional image of the density of electrons within the crystalcan be produced. From this electron density, the position of the atomsin the crystal can be determined, as well as their chemical bonds, theirdisorder, and other information. In particular, XRD can be used toestimate crystallite size; the peak width is inversely proportional tocrystallite size; as the crystallite size gets smaller, the peak getsbroader. In one or more embodiments, XRD is used to measure the averagecrystallite size of the CeO₂ particles.

The width of an XRD peak is interpreted as a combination of broadeningeffects related to both size and strain. The formulas used to determineboth are given below. The first equation below is the Scherrer equationwhich we use to transform full width at half maximum intensity, FWHM,information into a crystallite size for a given phase. The secondequation is used to calculate strain in a crystal from peak widthinformation and the total width or breadth of a peak considered to be asum of these two effects as shown in the third equation. It should benoticed that size and strain broadening vary in different fashions withregard to the Bragg angle θ. The constants for the Scherrer equation arediscussed below.

$\beta_{L} = \frac{K\;\lambda}{L\;\cos\;\theta}$ β_(e) = C ɛ tan  θ$\beta_{tot} = {{\beta_{e} + \beta_{L}} = {{C\; ɛ\;\tan\;\theta} + \frac{K\;\lambda}{L\;\cos\;\theta}}}$

The constants for the Scherrer equation are

K: shape constant, we use a value of 0.9

L: the peak width, this is corrected for the contribution from theinstrumental optics through the use of NIST SRM 660b LaB6 Line Position& Line Shape Standard

Θ: ½ of the 2θ value of the reflection of interest

λ: wavelength of radiation 1.5406 Å

Crystallite size is understood to be the length of the coherentscattering domain in a direction orthogonal to the set of lattice planeswhich give rise to the reflection. For CeO₂, the CeO₂ 111 reflection isthe most intense peak in the X-ray diffraction pattern of CeO₂. The CeO₂(111) plane of atoms intersects each of the crystallographic axes atunity and is orthogonal to the body diagonal represented by the <111>vector. So, a crystallite size of 312 Å calculated from the FWHM of theCeO₂ 111 reflection would be considered to be roughly 100 layers of the(111) plane of atoms.

Different directions, and thus reflections, in a crystal will generatedifferent though close crystallite size values. The values will be exactonly if the crystal is a perfect sphere. A Williamson Hall plot is usedto interpret size and strain effects by considering the total peakbreadth as a linear equation below with the slope of the linerepresenting strain and the intercept being the size of a crystal.

${\beta_{tot}\cos\;\theta} = {{C\; ɛ\;\sin\;\theta} + \frac{K\;\lambda}{L}}$

To determine the crystallite size of a material we need to determine theFWHM value of a single reflection or from the complete X-ray diffractionpattern. Traditionally we have fit a single reflection to determine theFWHM value of that reflection, corrected the FWHM value for thecontribution from the instrument, and then converted the corrected FWHMvalue into a crystallite size value using the Scherrer equation. Thiswould be done by ignoring any effect from strain in the crystal. We haveused this method primarily for questions concerning the crystallite sizeof precious metals for which we have only a single useful reflection. Itshould be noted that in fitting peaks it is desired to have a cleanreflection which is not overlapped by reflections from other phases.This is rarely the case with our present washcoat formulations so wehave shifted to using Rietveld methods. Rietveld methods allow us to fitcomplex X-ray diffraction patterns using the known crystal structures ofthe phases present. The crystal structures act as restraints or brakeson the fitting process. Phase content, lattice parameters, and FWHMinformation are varied for each phase until the overall model matchesthe experimental data.

In the Examples below, Rietveld methods were used to fit experimentalpatterns for fresh and aged samples. A FWHM curve determined for eachphase in each sample was used to determine a crystallite size. Straineffects were excluded.

As used herein, the term “alkaline earth metal” refers to one or morechemical elements defined in the Periodic Table of Elements, includingberyllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium(Ba), and radium (Ra). In one or more embodiments, the first layercomprises an alkaline earth metal. In one or more embodiments, thealkaline earth metal in the first layer can comprise beryllium (Be),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium(Ra). In specific embodiments, the alkaline earth metal in the firstlayer comprises barium. The alkaline earth metal can be present in thefirst layer in an amount in the range of about 5% to 30% by weight on anoxide basis, based on the weight of the first layer. In a specificembodiment, in the first layer, the alkaline earth metal comprisesbarium, which is present in an amount in the range of about 5% to about30% by weight on an oxide basis. In one or more embodiments, thealkaline earth metal can be incorporated into the layer as a salt (e.g.,BaCO₃).

In one or more embodiments, without intending to be bound by theory, itis thought that the additional ceria surface area resulting from smallercrystallite sizes allows for higher BaCO₃ based NO_(x) trapping due tobetter BaCO₃ dispersing, higher CeO₂ based NO_(x) trapping at lowtemperature, improved NO_(x) reduction due to more efficient WGS, andimproved NO oxidation and NO_(x) reduction due to better PGM dispersion.Thus, incorporating barium (BaCO₃) into ceria-alumina (CeO₂/Al₂O₃) has atremendous stabilization effect on CeO₂ and provides an LNT catalystmaterial in the first layer with improved hydrothermal stability, higherNO_(x) trapping capacity, and higher NO_(x) conversion than traditionalLNT technologies.

In one or more embodiments, the composite of CeO₂ and Al₂O₃ in the firstlayer contains ceria in an amount in the range of 20 to 80% by weight onan oxide basis, including 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%,65%, 70%, 75%, or 80%.

As used herein, the term “platinum group metal” or “PGM” refers to oneor more chemical elements defined in the Periodic Table of Elements,including platinum (Pt), palladium, rhodium, osmium, iridium, andruthenium, and mixtures thereof. In one or more embodiments, the firstlayer comprises at least one first platinum group metal supported on therare earth oxide-high surface area refractory metal oxide particles(e.g. ceria-alumina). In one or more embodiments, the first platinumgroup metal is selected from the group consisting of platinum,palladium, rhodium, and mixtures thereof. In a specific embodiment, thefirst platinum group metal component comprises both palladium andplatinum. In other embodiments, the first platinum group metal comprisesplatinum only. In a very specific embodiment, the first layer comprisesPt/Pd supported on BaCO₃/(CeO₂—Al₂O₃) particles. In another specificembodiment, the first layer comprises Pt supported on BaCO₃/(CeO₂—Al₂O₃)particles.

Generally, there are no specific restrictions as far as the platinum topalladium weight ratio of the first layer is concerned. Generally, thereare no specific restrictions as far as the palladium content of thefirst layer is concerned. There are also no specific restrictions as faras the platinum content of the first layer is concerned.

Generally, there are no specific restrictions as far as the totalplatinum group metal content of the layered catalyst composite isconcerned. In one or more embodiments, the first layer comprisesplatinum and palladium, and the second layer comprises palladium andrhodium. Generally, there are no specific restrictions as far as thetotal platinum group metal content of the layered catalyst composite isconcerned.

In one or more embodiments, the second layer comprises a second platinumgroup metal component supported on a first oxygen storage component(OSC) and/or a first refractory metal oxide support and, optionally, athird platinum group metal supported on a second refractory metal oxidesupport or a second oxygen storage component. In one or moreembodiments, the second platinum group metal component is selected fromplatinum, palladium, rhodium, or mixtures thereof. In specificembodiments, the second platinum group metal component comprisespalladium. Generally, there are no specific restrictions as far as thepalladium content of the second layer is concerned.

In one or more embodiments, the second layer does not comprise a thirdplatinum group metal. In one or more embodiments, when present, thethird platinum group metal is selected from platinum, palladium,rhodium, and mixtures thereof. In specific embodiments, the thirdplatinum group metal component comprises rhodium. Generally there are nospecific restrictions as far as the rhodium content of the second layeris concerned.

In one or more embodiments, the first layer comprises barium in amountin the range of about 5% to 30% by weight on an oxide basis of the firstlayer. In a specific embodiment, the first layer comprises Pt/Pdsupported on BaCO₃/(CeO₂—Al₂O₃) particles.

As used herein, the term “oxygen storage component” (OSC) refers to anentity that has a multi-valence state and can actively react withreductants such as carbon monoxide (CO) or hydrogen under reductionconditions and then react with oxidants such as oxygen or nitrous oxidesunder oxidative conditions. Examples of suitable oxygen storagecomponents comprise the rare earth oxides, particularly ceria. The OSCcan also comprise one or more of lanthana, praseodymia, neodynmia,niobia, europia, samaria, ytterbia, yttria, zirconia, and mixturesthereof in addition to ceria. The rare earth oxide may be in bulk (e.g.particulate) form. The oxygen storage component can include cerium oxide(ceria, CeO₂) in a form that exhibits oxygen storage properties. Thelattice oxygen of ceria can react with carbon monoxide, hydrogen, orhydrocarbons under rich A/F conditions. Upon lean exposure, the reducedceria has the ability to recapture oxygen from air and/or NO_(x)species, thus promoting conversion of NO_(x).

In one or more embodiments, the first and second oxygen storagecomponents comprise a ceria-zirconia composite or a rareearth-stabilized ceria-zirconia. In specific embodiments, the firstoxygen storage component and the second oxygen storage componentcomprise different ceria-zirconia composites. Specifically, the firstoxygen storage component comprises ceria in the range of 35 to 45% byweight and zirconia in the range of 43 to 53% by weight, and the secondoxygen storage component comprises ceria in the range of 15 to 25% byweight and zirconia in the range of 70 to 80% by weight.

According to one or more embodiments, the layered catalyst composite ofthe present invention is free of hydrocarbon trap material. As usedherein, the term “free of hydrocarbon trap material” means that nohydrocarbon trap material has been intentionally added to the layeredcatalyst composite. As used herein, the term “hydrocarbon trap material”refers to a material that has the ability to reversibly traphydrocarbons, particularly, hydrocarbon emissions produced during thecold start period. In one or more embodiments, the layered catalystcomposite contains less than 1% of hydrocarbon trap material.

Typically, the layered catalyst composite of the present invention isdisposed on a substrate. The substrate may be any of those materialstypically used for preparing catalysts, and will typically comprise aceramic or metal honeycomb structure. Any suitable substrate may beemployed, such as a monolithic substrate of the type having fine,parallel gas flow passages extending therethrough from an inlet or anoutlet face of the substrate, such that passages are open to fluid flowtherethrough (referred to herein as flow-through substrates). Thepassages, which are essentially straight paths from their fluid inlet totheir fluid outlet, are defined by walls on which the catalytic materialis coated as a washcoat so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels, which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular, etc.

Such monolithic substrates may contain up to about 900 or more flowpassages (or “cells”) per square inch of cross section, although farfewer may be used. For example, the substrate may have from about 7 to600, more usually from about 100 to 400, cells per square inch (“cpsi”).The cells can have cross sections that are rectangular, square,circular, oval, triangular, hexagonal, or are of other polygonal shapes.The ceramic substrate may be made of any suitable refractory material,e.g., cordierite, cordierite-alumina, silicon nitride, or siliconcarbide, or the substrates may be composed of one or more metals ormetal alloys.

The layered catalyst composite according to embodiments of the presentinvention can be applied to the substrate surfaces by any known means inthe art. For example, the catalyst washcoat can be applied by spraycoating, powder coating, or brushing or dipping a surface into thecatalyst composition.

In one or more embodiments, the layered catalyst composite is disposedon a honeycomb substrate.

The washcoat composition of this invention may be more readilyappreciated by reference to FIGS. 1 and 2. FIGS. 1 and 2 show arefractory substrate member 2, in accordance with one embodiment of thepresent invention. Referring to FIG. 1, the refractory substrate member2 is a cylindrical shape having a cylindrical outer surface 4, anupstream end face 6 and a downstream end face 8, which is identical toend face 6. Substrate member 2 has a plurality of fine, parallel gasflow passages 10 formed therein. As seen in FIG. 2 flow passages 10 areformed by walls 12 and extend through substrate from upstream end face 6to downstream end face 8, the passages 10 being unobstructed so as topermit the flow of a fluid, e.g., a gas stream, longitudinally throughsubstrate via gas flow passages 10 thereof. A discrete bottom layer 14,which in the art and sometimes below is referred to as a “washcoat”, isadhered or coated onto the walls 12 of the substrate member. As shown inFIG. 2, a second discrete top washcoat layer 16 is coated over thebottom washcoat layer 14. In one or more embodiments, the first layer isthe bottom washcoat layer 14, and the second layer is the top washcoatlayer 16. In other embodiments, the second layer is the bottom washcoatlayer 14, and the first layer is the top washcoat layer 16.

As shown in FIG. 2, the substrate member includes void spaces providedby the gas-flow passages 10, and the cross-sectional area of thesepassages 10 and the thickness of the walls 12 defining the passages willvary from one type of substrate member to another. Similarly, the weightof washcoat applied to such substrates will vary from case to case.Consequently, in describing the quantity of washcoat or catalytic metalcomponent or other component of the composition, it is convenient to useunits of weight of component per unit volume of substrate. Therefore,the units of grams per cubic inch (“g/in³”) and grams per cubic foot(“g/ft³”) are used herein to mean the weight of a component per volumeof the substrate member, including the volume of void spaces of thesubstrate member.

During operation, exhaust gaseous emissions from a lean burn enginecomprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfuroxides initially encounter the top washcoat layer 16, and thereafterencounter the bottom washcoat layer 14.

In one embodiment, the layered catalyst composite of the presentinvention comprises two distinct layers coated on a single substrate orsubstrate member, one layer (e.g., the second layer) over top of theother (e.g., the first layer). In this embodiment, the first layer iscoated over the entire axial length of a substrate (e.g., a flow-throughmonolith) and the second layer is coated over the entire axial length ofthe first layer.

In one or more embodiments, the improved NO_(x) conversion upon hightemperature severe aging allows the placement of the layered catalystcomposite according to one or more embodiments in a close-coupledposition, which is beneficial for reducing system N₂O emissions becauseN₂O formation decreases with temperature increasing.

According to one or more embodiments, the layered catalyst composite iseffective to provide both lean NO_(x) trap (LNT) functionality andthree-way conversion (TWC) functionality. As used herein, the term“conversion” encompasses both the chemical conversion of emissions toother compounds, as well as the trapping of emissions by chemical and/oradsorptive binding to an appropriate trapping material. As used herein,the term “emissions” refers to exhaust gas emissions, more specificallyto exhaust gas emissions comprising NO_(x), CO, and hydrocarbons.

In a specific embodiment, the substrate comprises a flow-throughcarrier, and the first layer is disposed on the substrate, and thesecond layer is disposed on top of the first layer. The first layercomprises ceria-alumina particles having a ceria phase present in aweight percent of the composite in the range of about 20% to about 80%on an oxide basis, the ceria-alumina particles having barium supportedon the particles, and platinum and palladium supported thereon. Thesecond layer comprises Pd supported on a ceria-zirconia composite and Rhsupported on alumina.

Catalytic converters must effectively convert hydrocarbons at lowtemperatures during lean operation. The same also applies tohydrocarbons (HCs) and nitrogen oxides (NO_(x)) under conditionsfavoring stoichiometric exhaust gas, which can occur during thecold-start phase as well as during operation. An additional challenge isstoring nitrogen oxides during lean combustion and reducing these oxidesduring rich combustion. In order to utilize lean combustion as far aspossible, NO_(x) storage must be possible in a large temperature range.

Thus, the layered catalyst composite of the present invention iseffective to provide both lean NO_(x) trap functionality (LNT) andthree-way conversion (TWC) functionality. In one or more embodiments,the layered catalyst composite of the present invention is effective tosimultaneously store NO_(x), and to oxidize CO, HC, and NO to NO₂.According to one or more embodiments, under rich conditions, the layeredcatalyst composite is effective to simultaneously convert CO and HC andto release and reduce NO_(x), and under stoichiometric conditions, thelayered catalyst composite is effective to simultaneously convert CO,HC, and NO_(x).

Occasionally, particulates are present in the exhaust gas stream and thelayered catalyst composites may also provide the ability to oxidize anyparticulates.

The layered catalyst composite of the present invention can be used inan integrated emission treatment system comprising one or moreadditional components for the treatment of exhaust gas emissions. Thus,a second aspect of the present invention is directed to an exhaust gastreatment system. In one or more embodiments, the exhaust gas treatmentsystem comprises an engine and the layered catalyst composite of thepresent invention. In specific embodiments, the engine is a lean burnengine. In other specific embodiments, the engine is a lean gasolinedirect injection engine. In one or more embodiments, the exhaust gastreatment system comprises a lean burn engine upstream from the layeredcatalyst composite of one or more embodiments. The exhaust gas treatmentsystem may further comprise a catalyst, and optionally, a particulatefilter. In one or more embodiments, the catalyst is selected from thegroup consisting of TWC, SCR, GPF, LNT, AMOx, SCR on a filter, andcombinations thereof. The layered catalyst composite can be locatedupstream or downstream of the catalyst. In one or more embodiments, thecatalyst is a SCR catalyst located downstream of the layer catalystcomposite. In one or more embodiments, the particulate filter can beselected from a gasoline particulate filter, a soot filter, or a SCR ona filter. The particulate filter may be catalyzed for specificfunctions. The layered catalyst composite can be located upstream ordownstream of the particulate filter.

In a specific embodiment, the particulate filter is a catalyzed sootfilter (CSF). The CSF can comprise a substrate coated with a washcoatlayer containing one or more catalysts for burning off trapped soot andor oxidizing exhaust gas stream emissions. In general, the soot burningcatalyst can be any known catalyst for combustion of soot. For example,the CSF can be coated with a one or more high surface area refractoryoxides (e.g., alumina, silica, silica alumina, zirconia, and zirconiaalumina) and/or an oxidation catalyst (e.g., a ceria-zirconia) for thecombustion of unburned hydrocarbons and to some degree particulatematter. In one or more embodiments, the soot burning catalyst is anoxidation catalyst comprising one or more precious metal (PM) catalysts(platinum, palladium, and/or rhodium).

In general, any known filter substrate in the art can be used,including, e.g., a honeycomb wall flow filter, wound or packed fiberfilter, open-cell foam, sintered metal filter, etc., with wall flowfilters being specifically exemplified. Wall flow substrates useful forsupporting the CSF compositions have a plurality of fine, substantiallyparallel gas flow passages extending along the longitudinal axis of thesubstrate. Typically, each passage is blocked at one end of thesubstrate body, with alternate passages blocked at opposite end-faces.Such monolithic substrates may contain up to about 900 or more flowpassages (or “cells”) per square inch of cross section, although farfewer may be used. For example, the substrate may have from about 7 to600, more usually from about 100 to 400, cells per square inch (“cpsi”).The porous wall flow filter used in embodiments of the invention isoptionally catalyzed in that the wall of said element has thereon orcontained therein one or more catalytic materials, such CSF catalystcompositions are described hereinabove. Catalytic materials may bepresent on the inlet side of the element wall alone, the outlet sidealone, both the inlet and outlet sides, or the wall itself may consistall, or in part, of the catalytic material. In another embodiment, thisinvention may include the use of one or more washcoat layers ofcatalytic materials and combinations of one or more washcoat layers ofcatalytic materials on the inlet and/or outlet walls of the element.

A third aspect of the present invention is directed to a method oftreating a gas comprising hydrocarbons (HC), carbon monoxide (CO), andnitrogen oxides (NO_(x)). In one or more embodiments, the methodcomprises contacting the gas with the layered catalyst composite of thepresent invention. In specific embodiments, under lean conditions, thelayered catalyst composite is effective to simultaneously store NO_(x),and to oxidize CO, HC, and NO; under rich conditions, the layeredcatalyst composite is effective to simultaneously convert CO and HC andto release and reduce NO_(x); and under stoichiometric conditions, thelayered catalyst composite is effective to simultaneously convert CO,HC, and NO_(x).

A further aspect of the present invention is directed to a method ofmaking a layered catalyst composite. In one or more embodiments, themethod comprises providing a carrier and coating the carrier with firstand second layers of catalytic material. In specific embodiments, thefirst layer comprises rare earth oxide-high refractory metal oxideparticles, an alkaline earth metal supported on the rare earthoxide-high refractory metal oxide particles, and at least one firstplatinum group metal component supported on the rare earth oxide-highrefractory metal oxide particles. The second layer, being the outermostlayer of the composite, comprises a second platinum group metalcomponent supported on a first oxygen storage component (OSC) or a firstrefractory metal oxide support and a third platinum group metalcomponent supported on a second refractory metal oxide support or asecond oxygen storage component.

The invention is now described with reference to the following examples.Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

EXAMPLES Example 1 Preparation of LNT-TWC Catalyst

To demonstrate the advantage of this invention, an example of a LNT-TWCcatalyst was prepared. This two layer formulation, which comprises anundercoat washcoat layer and a top washcoat layer, was coated onto aflow-through ceramic monolith substrate carrier having a cell density of400 cells per square inch (cpsi) and a 4 mil wall thickness, the topwashcoat layer being coated over the undercoat washcoat layer. Thecatalyst has a total 185 g/ft³ PGM nominal loading with a Pt/Pd/Rh ratioof 63/117/5.

Undercoat Washcoat Layer

The CeO₂—Al₂O₃ particles comprising 50 wt. % of CeO₂ and 50 wt. % ofAl₂O₃ were impregnated with a solution of barium acetate such that theBaCO₃/(CeO₂—Al₂O₃) composite had a BaCO₃ content of about 26 wt. %. Themixture was dried at 110° C. and calcined at 720° C. for 2 hours. Pd inthe form of palladium nitrate and Pt in the form of platinum aminesolution were introduced onto the support material BaCO₃/(CeO₂—Al₂O₃) byconventional incipient wetness impregnation. A slurry mixture containingabout 87 wt. % of BaCO₃/(CeO₂—Al₂O₃), 1 wt. % of Pt, 0.1 wt. % of Pd,magnesium acetate to yield 7 wt. % of MgO, zirconium acetate to yield 4wt. % of ZrO₂, was coated onto ceramic honeycomb substrates. The totalwashcoat loading of the undercoat layer after 550° C. calcination forone hour in air was about 3.4 g/in³.

Topcoat Layer

The top layer was disposed on the undercoat layer. Pd in the form ofpalladium nitrate was introduced onto the OSC material and Rh in theform of rhodium nitrate was introduced onto the activated γ-alumina. Aslurry mixture containing about 15 wt. % of activated γ-alumina, 76 wt.% of OSC material (CeO₂/ZrO₂) with promoters, 2 wt. % of Pd, 0.1 wt. %of Rh, barium acetate to yield 5 wt. % of BaCO₃, zirconium acetate toyield 2 wt. % of ZrO₂, was coated over the entire undercoat layer. Thetotal washcoat of the top layer after 550° C. calcination was about 3.0g/in³.

Example 2 Preparation of LNT-TWC Catalyst

To demonstrate the advantage of this invention, an example of a LNT-TWCcatalyst was prepared. This two layer formulation, which comprises anundercoat washcoat layer and a top washcoat layer, was coated onto aflow-through ceramic monolith substrate carrier having a cell density of400 cells per square inch (cpsi) and a 4 mil wall thickness, the topwashcoat layer being coated over the undercoat washcoat layer. Thecatalyst has a total 185 g/ft3 PGM nominal loading with a Pt/Pd/Rh ratioof 63/117/5.

Undercoat Washcoat Layer

The CeO₂—Al₂O₃ particles comprising 30 wt. % of CeO₂ and 70 wt. % ofAl₂O₃ were impregnated with a solution of barium acetate such that theBaCO₃/(CeO₂—Al₂O₃) composite had a BaCO₃ content of about 13 wt. %. Themixture was dried at 110° C. and calcined at 720° C. for 2 hours. Pd inthe form of palladium nitrate and Pt in the form of platinum aminesolution were introduced onto the support material BaCO₃/(CeO₂—Al₂O₃) byconventional incipient wetness impregnation. A slurry mixture containingabout 87 wt. % of BaCO₃/(CeO₂—Al₂O₃), 1 wt. % of Pt, 0.1 wt. % of Pd,magnesium acetate to yield 7 wt. % of MgO, zirconium acetate to yield 4wt. % of ZrO₂, was coated onto ceramic honeycomb substrates. The totalwashcoat loading of the undercoat layer after 550° C. calcination forone hour in air was about 3.4 g/in³.

Topcoat Layer

The top layer was disposed on the undercoat layer. Pd in the form ofpalladium nitrate was introduced onto the OSC material and Rh in theform of rhodium nitrate was introduced onto the activated γ-alumina. Aslurry mixture containing about 18 wt. % of activated γ-alumina, 70 wt.% of OSC material (CeO₂/ZrO₂) with promoters, 2.7 wt. % of Pd, 0.1 wt. %of Rh, barium acetate to yield 8.6 wt. % of BaCO₃, zirconium acetate toyield 2 wt. % of ZrO₂, was coated over the entire undercoat layer. Thetotal washcoat of the top layer after 550° C. calcination was about 2.4g/in³.

Example 3 Preparation of LNT Catalyst (Comparative)

To demonstrate the advantage of this invention, a comparative example ofa state-of-art LNT catalyst was prepared. This two layer formulation,which comprises an undercoat washcoat layer and a top washcoat layer,was coated onto a flow-through ceramic monolith substrate carrier havinga cell density of 400 cells per square inch (cpsi) and a 4 mil wallthickness, the top washcoat layer being coated over the undercoatwashcoat layer. The catalyst has a total 120 g/ft³ PGM nominal loadingwith a Pt/Pd/Rh ratio of 103/12/5, which is cost equivalent to Examples1, 2, and 7 at 185 g/ft³ PGM nominal loading with a Pt/Pd/Rh ratio of63/117/5.

The undercoat layer contains an activated γ-alumina, cerium oxide,barium carbonate, magnesia, zirconia, platinum, and palladium atconcentrations of approximately 38%, 41%, 14%, 6%, 2%, 0.7% and 0.09%,respectively, based on the calcined weight of the catalyst. Pd in theform of palladium nitrate and Pt in the form of platinum amine solutionwere introduced onto the support material by conventional incipientwetness techniques. The total washcoat loading of the undercoat layerafter 550° C. calcination for one hour in air was about 5.3 g/in³.

The topcoat layer, which is disposed on the undercoat layer, contains anactivated γ-alumina, cerium oxide, platinum, palladium and rhodium atconcentrations of approximately 57%, 41%, 2%, 0.2 and 0.2%,respectively, based on the calcined weight of the catalyst. Pt in theform of platinum amine solution and Pd in the form of palladium nitratesolution were introduced onto γ-alumina, and Rh in the form of rhodiumnitrate was introduced onto ceria by conventional incipient wetnesstechniques. The topcoat layer was coated over the entire undercoatlayer. The total washcoat of the topcoat layer after 550° C. calcinationwas about 1.23 g/in³.

Example 4 Preparation of TWC Catalyst (Comparative)

To demonstrate the advantage of this invention, a comparative example ofa state-of-art TWC catalyst was prepared. This single layer formulationwas coated onto a flow-through ceramic monolith substrate carrier havinga cell density of 400 cells per square inch (cpsi) and a 4 mil wallthickness. The catalyst has a total 248 g/ft³ PGM nominal loading with aPt/Pd/Rh ratio of 24/220/4, which is cost equivalent to Examples 1, 2,and 7 at 185 g/ft³ PGM nominal loading with a Pt/Pd/Rh ratio of63/117/5.

The catalyst washcoat contains an activated γ-alumina, OSC material(CeO₂/ZrO₂) with promoters, barium carbonate, zirconia, platinum,palladium and rhodium at concentrations of approximately 36%, 56%, 4%,1%, 0.3%, 3% and 0.06%, respectively, based on the calcined weight ofthe catalyst. The total washcoat loading after 550° C. calcination forone hour in air was about 4.0 g/in³.

Example 5 Cycle NO_(x) Conversion and NO_(x) Trapping Capacity Testing

NO_(x) trapping and reduction activity of Examples 1, 2, and 3 wereevaluated in fresh and after aging at 950° C. for 5 hours in 2% O₂ and10% steam in N₂. The catalysts were evaluated on a reactor test rig withFTIR analytical apparatus. The evaluations were conducted with 10 cyclescomprising a 120 seconds lean gas exposure and a 5 seconds rich gasexposure. A purging with a gas mixture of CO₂, H₂O and N₂ is appliedbetween lean gas exposure and rich gas exposure for the evaluations at200, 250, 300, 350, and 400° C. in 10, 10, 6, 4, and 4 seconds,respectively. After lean/rich cycles, the catalyst was regenerated inrich gas for 1 minute, and then exposed to lean gas. The feeding gascompositions and space velocities at each testing temperatures arelisted in Table 1.

TABLE 1 Temperature, ° C. 200 and 250 300 350 400 450 500 SV, hr⁻¹25,000 40,000 55,000 70,000 55,000 80,000 Lean Rich Lean Rich Lean RichLean Rich Lean Rich Lean Rich O₂, % 13 0 11 0 11 0 6 0 11 0 6 0 CO₂, %4.15 4.15 4.15 4.15 5 5 5 5 5 5 5 5 NO, ppm 300 0 300 0 300 0 300 0 3000 300 0 CO/H₂ 0 4.5 0 4.5 0 4.5 0 4.5 0 4.5 0 4.5 (3:1), % HC*, ppm 1001000 100 1000 100 1000 100 1000 100 1000 100 1000 H₂O, % 8 8 8 8 8 8 8 88 8 8 8

The NO_(x) trapping capacity of the catalyst was measured after the endof the 1 minute rich exposure and presented as the amount of NO_(x)removed from the feeding gas when 100 ppm of NO_(x) was released. Thecycle NO_(x) conversion of the catalyst was measured as an averageNO_(x) conversion of the last five lean/rich cycles.

${{Cycle}\mspace{14mu}{NOx}\mspace{14mu}{Conversion}\mspace{11mu}(\%)} = {\frac{( {{{NOx}\mspace{20mu}{input}} - {{NOx}\mspace{20mu}{output}}} )}{{NOx}\mspace{14mu}{input}} \times 100\;\%}$

Example 6 XRD Measurement

The CeO₂ crystallite size of the Example 1 and 3 aged samples wasmeasured by XRD. The samples were ground using a mortar and pestle andthen packed onto a low background slide for analysis. A PANalytical MPDX'Pert Pro diffraction system was used to collect data in Bragg-Brentanogeometry. We used Cu_(Kα) radiation in the analysis with generatorsettings of 45kV and 40 mA. The optical path consisted of a ¼°divergence slit, 0.04 radian soller slits, 15 mm mask, ½° anti-scatterslit, ¼° anti-scatter slit, Ni filter, and X'Celerator linear positionsensitive detector. Data was collected from 10° to 90° 2θ using a stepsize of 0.026° 2θ and a count time of 600 s per step. Jade Plus 9analytical X-ray diffraction software was used for phase identification.The phase present was identified by search/match of the PDF-4/Full Filedatabase from ICDD, which is the International Center for DiffractionData. All numerical values were determined using Rietveld methods.

The LNT-TWC catalyst Examples 1 and 2 significantly improved cycleNO_(x) conversion and NO_(x) trapping capacity relative to the TWCcatalyst Comparative Example 4, as presented in FIGS. 3A and 3B.Although the cycle NO_(x) conversion and the NO_(x) trapping capacity ofExamples 1 and 2 in fresh state are not as high as those of the LNTcatalyst Example 3 in fresh state, Examples 1 and 2 have higherhydrothermal stability than Example 3. As presented in FIGS. 4A and 4B,after aging at 950° C. for 5 hours in 2% O₂ and 10% steam in N₂,Examples 1 and 2 show higher cycle NO_(x) conversion and NO_(x) trappingcapacity than Example 3.

The high hydrothermal stability of Example 1 was also demonstrated byaverage CeO₂ crystallite size as measured by XRD after aging at 950° C.for 5 hours in 2% O₂ and 10% steam in N₂. The results are presented inTable 2. The CeO₂ present in Example 1 is more hydrothermally stablethan that in Example 3. The average CeO₂ crystallite size of Example 1is 109 Å after aging at 950° C. for 5 hours in 2% O₂ and 10% steam inN₂. The average CeO₂ crystallite size of Example 3 is 197 Å after at950° C. for 5 hours in 2% O₂ and 10% steam in N₂. This stabilizationeffect is likely beneficial for NO_(x) trapping and NO_(x) reductionactivity. The additional ceria surface area resulting from smallercrystallite sizes will allow for more low temperature ceria based NO_(x)trapping, improve WGS, and improve PGM dispersion.

TABLE 2 Example CeO₂ Crystallite Size (Å)* Example 1 109 ComparativeExample 3 197 *Measured after aging at 950° C. for 5 hours in 2% O₂ and10% steam in N₂

Rietveld methods were used to fit experimental patterns for the agedExample 1 and Example 3. A FWHM curve determined for each phase in eachsample was used to determine a crystallite size. Strain effects wereexcluded.

Examples 1 and 4, respectively, were applied to treat the exhaust gasstream of a lean-burn gasoline engine after aging at 950° C. for 64hours in an internal combustion engine placed downstream of a TWCcatalyst. As presented in FIGS. 5A and 5B, in a FTP75 testing cycle,Example 1, when placed downstream of a TWC catalyst, significantlyreduced NO_(x) emissions relative to Example 4, when placed downstreamof the same TWC catalyst, and Example 1 showed equivalent non-methanehydrocarbon (NMHC) emissions to Example 4.

TEM of the undercoat layer of Example 1 showed that plates of Al₂O₃ andround agglomerates of CeO₂ are intimately mixed, and the nano-sizedplatinum particles are located on the mixed CeO₂ and Al₂O₃ particles, aspresented in FIG. 6.

TEM of the topcoat layer of Example 1 showed that Rh particles arelocated on Al₂O₃ and Pd particles are located on OSC material, aspresented in FIG. 7

Example 7

To demonstrate the advantage of this invention, an example of a LNT-TWCcatalyst was prepared. This two layer formulation, which comprises anundercoat washcoat layer and a top washcoat layer, was coated onto aflow-through ceramic monolith substrate carrier having a cell density of400 cells per square inch (cpsi) and a 4 mil wall thickness, the topwashcoat layer being coated over the undercoat washcoat layer. Thecatalyst has a total 185 g/ft³ PGM nominal loading with a Pt/Pd/Rh ratioof 63/117/5.

Undercoat Washcoat Layer

Pd in the form of palladium nitrate was introduced onto the OSC materialand Rh in the form of rhodium nitrate was introduced onto the activatedγ-alumina. A slurry mixture containing about 15 wt. % of activatedγ-alumina, 80 wt. % of OSC material (CeO₂/ZrO₂) with promoters, 2.3 wt.% of Pd, 0.1 wt. % of Rh, zirconium acetate to yield 2 wt. % of ZrO₂,was coated onto ceramic honeycomb substrates. The total washcoat of thetop layer after 550° C. calcination was about 2.8 g/in³.

Topcoat Layer

The top layer was disposed on the undercoat layer. The CeO₂—Al₂O₃particles comprising 50 wt. % of CeO₂ and 50 wt. % of Al₂O₃ wereimpregnated with a solution of barium acetate such that theBaCO₃/(CeO₂—Al₂O₃) composite had a BaCO₃ content of about 26 wt. %. Themixture was dried at 110° C. and calcined at 720° C. for 2 hours. Pd inthe form of palladium nitrate and Pt in the form of platinum aminesolution were introduced onto the support material BaCO₃/(CeO₂—Al₂O₃) byconventional incipient wetness impregnation. A slurry mixture containingabout 87 wt. % of BaCO₃/(CeO₂—Al₂O₃), 1 wt. % of Pt, 0.1 wt. % of Pd,magnesium acetate to yield 7 wt. % of MgO, zirconium acetate to yield 4wt. % of ZrO₂, was coated over the entire under coat layer. The totalwashcoat loading of the undercoat layer after 550° C. calcination forone hour in air was about 3.4 g/in³.

What is claimed is:
 1. A layered catalyst composite for an exhauststream of an internal combustion engine, the layered catalyst compositecomprising a catalytic material on a substrate, the catalytic materialcomprising at least two layers, wherein: the first layer comprises rareearth oxide-high surface area refractory metal oxide particles, analkaline earth metal supported on the rare earth oxide-high surface arearefractory metal oxide particles, and at least one first platinum groupmetal component supported on the rare earth oxide-high surface arearefractory metal oxide particles, wherein the rare earth oxide-highsurface area refractory metal oxide particles have a ceria phase presentin a weight percent of the particles in the range of about 20% to about80% on an oxide basis; and the second layer comprises a second platinumgroup metal component supported on a first oxygen storage component(OSC) and/or a first refractory metal oxide support and, optionally, athird platinum group metal supported on a second refractory metal oxidesupport or a second oxygen storage component.
 2. The layered catalystcomposite of claim 1, wherein the catalyst is effective to provide bothlean NO_(x) trap functionality and three-way conversion functionality.3. The layered catalyst composite of claim 1, wherein the first layer isdisposed on the substrate that comprises a flow-through monolith and thesecond layer is disposed on the first layer.
 4. The layered catalystcomposite of claim 1, wherein the second layer is disposed on thesubstrate that comprises a flow-through monolith and the first layer isdisposed on the second layer.
 5. The layered catalyst composite of claim1, wherein the substrate comprises a wall-flow filter and the firstlayer is on an inlet set of passages and the second layer is on anoutlet set of passages.
 6. The layered catalyst composite of claim 1,wherein the substrate comprises a wall-flow filter and the first layeris on an outlet set of passages and the second layer is on an inlet setof passages.
 7. The layered catalyst composite of claim 1 that is freeof hydrocarbon trap material.
 8. The layered catalyst composite of claim1, wherein the rare earth oxide-high surface area refractory metal oxideparticles comprise ceria-alumina particles.
 9. The layered catalystcomposite of claim 8, wherein the ceria-alumina particles aresubstantially free of alkaline earth metal.
 10. The layered catalystcomposite of claim 1, wherein the first, second, and third platinumgroup metal components independently comprise platinum, palladium,and/or rhodium.
 11. The layered catalyst composite of claim 1, whereinthe first platinum group metal component comprises both palladium andplatinum.
 12. The layered catalyst composite of claim 1, wherein thefirst platinum group metal component comprises platinum.
 13. The layeredcatalyst composite of claim 1, wherein the second platinum group metalcomponent comprises palladium.
 14. The layered catalyst composite ofclaim 1, wherein the third platinum group metal component comprisesrhodium.
 15. The layered catalyst composite of claim 1, wherein thefirst and second refractory metal oxide supports independently comprisea compound that is activated, stabilized, or both selected from thegroup consisting of alumina, zirconia, alumina-zirconia,lanthana-alumina, lanthana-zirconia-alumina, baria-alumina,baria-lanthana-alumina, baria-lanthana-neodymia-alumina,alumina-chromia, alumina-ceria, and combinations thereof.
 16. Thelayered catalyst composite of claim 1, wherein the first and secondoxygen storage components comprise a ceria-zirconia composite or a rareearth-stabilized ceria-zirconia.
 17. The layered catalyst composite ofclaim 1, wherein the first oxygen storage component and the secondoxygen storage component comprise different ceria-zirconia composites,the first oxygen storage component comprising ceria in the range of 35to 45% by weight and zirconia in the range of 43 to 53% by weight andthe second oxygen storage component comprising ceria in the range of 15to 25% by weight and zirconia in the range of 70 to 80% by weight. 18.The layered catalyst composite of claim 1, wherein the alkaline earthmetal comprises barium.
 19. The layered catalyst composite of claim 18,wherein the barium is present in an amount in the range of about 5% to30% by weight on an oxide basis of the first layer.
 20. The layeredcatalyst composite of claim 19, wherein the barium is present in anamount in the range of about 0% to about 10% by weight on an oxide basisof the second layer.
 21. The layered catalyst composite of claim 18,wherein the second alkaline earth metal comprises barium.
 22. Thelayered catalyst composite of claim 1, wherein the second layer furthercomprises a second alkaline earth metal supported on the firstrefractory metal oxide support.
 23. The layered catalyst composite ofclaim 1, wherein under lean conditions, the layered catalyst compositeis effective to simultaneously store NO_(x), and to oxidize CO, HC, andNO to NO₂.
 24. The layered catalyst composite of claim 1, wherein underrich conditions, the layered catalyst composite is effective tosimultaneously convert CO and HC and to release and reduce NO_(x). 25.The layered catalyst composite of claim 1, wherein under stoichiometricconditions, the layer catalyst composite is effective to simultaneouslyconvert CO, HC, and NO_(x).
 26. The layered catalyst composite of claim1, wherein the catalyst composite is effective to provide both leanNO_(x) trap functionality and three-way conversion functionality; thesubstrate comprises a flow-through carrier and the first layer isdisposed on the substrate and the second layer is disposed on the firstlayer; the rare earth oxide-high surface area refractory metal oxideparticles comprise ceria-alumina particles; the first platinum groupmetal component comprises palladium and/or platinum; the second platinumgroup metal component comprises palladium; and the third platinum groupmetal component comprises rhodium.
 27. A method for treating a gascomprising hydrocarbons, carbon monoxide, and nitrogen oxidescomprising: contacting the gas with the layered catalyst composite ofclaim 1, wherein: under lean conditions, the layered catalyst compositeis effective to simultaneously store NO_(x), and to oxidize CO, HC, andNO; under rich conditions, the layered catalyst composite is effectiveto simultaneously convert CO and HC and to release and reduce NO_(x);and under stoichiometric conditions, the layered catalyst composite iseffective to simultaneously convert CO, HC, and NO_(x).
 28. A method ofmaking a layered catalyst composite, the method comprising providing acarrier and coating the carrier with first and second layers ofcatalytic material; the first layer comprising rare earth oxide-highsurface area refractory metal oxide particles, an alkaline earth metalsupported on the rare earth oxide-high surface area refractory metaloxide particles, and at least one first platinum group metal componentsupported on the rare earth oxide-high surface area refractory metaloxide particles, wherein the rare earth oxide-high surface arearefractory metal oxide particles have a ceria phase present in a weightpercent of the particles in the range of about 20% to about 80% on anoxide basis, the second layer being the outermost layer of thecomposite, comprising a second platinum group metal component supportedon a first oxygen storage component (OSC) or a first refractory metaloxide support and a third platinum group metal component supported on asecond refractory metal oxide support or a second oxygen storagecomponent.